1. Field of the Invention
The present invention relates to an expansion valve constituting a refrigerating cycle.
2. Description of the Related Art
There are expansion valves of various types. In widely used expansion valves, a valve plug is opposed downstream to an orifice that is formed by constricting the middle of a high-pressure refrigerant passage through which a high-pressure refrigerant to be fed into an evaporator passes. The valve plug is opened and closed in accordance with the temperature and pressure of a low-pressure refrigerant that is delivered from the evaporator.
Such an expansion valve is used in a refrigerating cycle 1, e.g., a vehicular air conditioning system shown in FIG. 21. The refrigerating cycle 1 comprises a refrigerant compressor 2 that is driven by means of an engine, a condenser 3 connected to the discharge side of the compressor 2, and a liquid reservoir 4 connected to the condenser 3. The cycle 1 further comprises an expansion valve 5, which adiabatically expands a liquid refrigerant from the reservoir 4 into a gas-liquid refrigerant, and an evaporator 6 connected to the valve 5. The expansion valve 5 is situated in the refrigerating cycle 1.
The expansion valve 5 is provided with a high-pressure passage 5b, through which the liquid refrigerant flows into a valve body 5a, and a low-pressure passage 5c through which the adiabatically expanded gas-liquid refrigerant flows out. The passages 5b and 5c communicate with each other by means of an orifice 7. A valve chest 8d of the valve 5 is provided with a valve plug 8, which adjusts the volume of passage of the refrigerant through the orifice 7.
Further, a low-pressure refrigerant passage 5d penetrates the valve body 5a of the expansion valve 5. A plunger 9a is disposed for sliding motion in the refrigerant passage 5d. The plunger 9a is driven by means of a temperature sensing drive unit 9, which is fixed to the upper part of the valve body 5a. The interior of the drive unit 9 is divided into two parts, an upper airtight chamber 9c and a lower airtight chamber 9c′, by a diaphragm 9d. A disc portion 9e on the upper end of the plunger 9a is in contact with the diaphragm 9d. 
At the bottom of the valve body 5a, moreover, a compression coil spring 8a that urges a support member 8c to press the valve plug 8 in the valve closing direction is located in the valve chest 8d. The valve chest 8d is defined by an adjusting screw 8b that mates with the valve body 5a, and is kept airtight by means of an O-ring 8e. An operating rod 9b, which moves the valve plug 8 in the valve opening direction as the plunger 9a slides, abuts against the lower end of the plunger 9a. 
The plunger 9a in the temperature sensing drive unit 9 transmits the temperature in the low-pressure refrigerant passage 5d to the upper airtight chamber 9c. The pressure in the chamber 9c changes depending on the transmitted temperature. If the temperature is high, for example, the pressure in the upper airtight chamber 9c rises, and the diaphragm 9d depresses the plunger 9a. Thereupon, the valve plug 8 moves in the valve opening direction, so that the volume of passage of the refrigerant through the orifice 7 increases, and the temperature of the evaporator 6 is lowered.
If the temperature in the low-pressure refrigerant passage 5d is low, on the other hand, the pressure in the upper airtight chamber 9c lowers, so that the force of the diaphragm 9d to depress the disc portion 9e lessens. Thereupon, the compression coil spring 8a, which presses the valve plug 8 in the valve closing direction, urges the valve plug 8 to move in the valve closing direction. Thus, the volume of passage of the refrigerant through the orifice 7 is reduced, and the temperature of the evaporator 6 is raised.
Thus, in the expansion valve 5, the valve plug 8 is moved to change the opening area of the orifice 7 in response to a change of temperature in the low-pressure refrigerant passage 5d. By doing this, the volume of passage of the refrigerant is regulated to adjust the temperature of the evaporator 6. According to the expansion valve 5 of this type, the opening area of the orifice 7 that adiabatically expands the liquid refrigerant into the gas-liquid refrigerant is set by adjusting the spring load of the variable-load compression coil spring 8a, which presses the valve plug 8 in the valve closing direction, by means of the adjusting screw 8b. 
In some cases, however, the high-pressure refrigerant that is fed into the expansion valve may undergo fluctuation in pressure on the upper-stream side in the refrigerating cycle. This pressure fluctuation is transmitted to the expansion valve through the medium of the high-pressure refrigerant.
If the refrigerant pressure on the upper-stream side is transmitted to the valve plug by the pressure fluctuation in the conventional expansion valve constructed in this manner, the action of the valve plug may possibly be destabilized. In this case, the expansion valve may fail to enjoy accurate flow control, or noise may be produced owing to vibration of the valve plug.
According to conventional means to solve this problem (see Jpn. Pat. Appln. KOKAI Publication No. 2001-50617), a spring or the like is used to apply an urging force laterally to an axially movable rod that is located between a power element and a valve plug, thereby preventing the valve plug from becoming susceptible to the pressure fluctuation of the high-pressure refrigerant so that its action is stable.